Vacuum process chambers are often employed in manufacturing to provide a vacuum environment for tasks such as semiconductor wafer fabrication, deposition processes, electron microscopy, gas chromatography, and others. Such chambers are typically achieved by attaching a vacuum pump to the vacuum process chamber in a sealed arrangement. The vacuum pump operates to remove substantially all of the molecules from the vacuum process chamber, therefore creating a vacuum environment.
One type of vacuum pump is a cryopump, such as that disclosed in U.S. Pat. No. 5,862,671, issued Jan. 26, 1999, assigned to the assignee of the present application and incorporated by reference in its entirety. Cryopumps remove molecules from a vacuum process chamber by cooling a surface to temperatures approaching absolute zero. At such temperatures, gases condense or are adsorbed on the cooled surface, called a cryogenic array, thereby removing molecules from the vacuum process chamber. The resulting vacuum chamber
Other types of cryopumps are designed to operate at temperatures greater than absolute zero. These cryopumps remove specific gases from a chamber such as water, hydrocarbons, process by-products, outgas species and process gases. Waterpumps as described on U.S. Pat. No. 5,483,803 is an example of one of such cryopumps. Other cryopumps such as that disclosed in U.S. Pat. No. 5,211,022 do not to remove all gases but maintain a low pressure of a process gas in a vacuum chamber.
Cryopumps typically employ a refrigerator to achieve the cryogenic temperatures required. The type of refrigerator required depends on the temperature required for the species being pumped and other parameters such as heat load and vibration. Typically, Stirling, Gifford-McMahon, and pulse tube refrigerators are used for cryogenic vacuum pumps. These refrigerators require a supply of compressed gas from a compressor to supply a flow of refrigerant to the cryogenic refrigerator in the cryopump. Cryopumps that require temperatures near absolute zero use helium as the compressed gas. A cryogenic array is in thermal communication with the cold end of the refrigerator and cooled therewith. Inside the refrigerator a displacer, driven by a displacer drive which reciprocates the displacer, regulates the quantity of helium used. Expansion of refrigerant gas, such as helium, in the refrigerator creates cooling and heat is drawn off the cryogenic array, generating the cryogenic temperatures required to condense gases on the cryogenic array.
Alternatively, pulse tube displacers do not move, but, rather, use a pressure wave instead. Although cryopumps have been described with motor driven designs, cryopumps may be designed with pneumatic driven systems.
The amount of helium refrigerant available to the cryogenic refrigerator determines the rate at which cooling occurs. A greater supply of helium enables the refrigerator to consume more helium which produces more refrigeration. It decreases the amount of time required for cool down, which is the time required to achieve cryopumping temperatures, and at operating temperatures enables the refrigerator to produce more refrigeration when demanded by the varying process conditions in the vacuum chamber. A greater supply of helium also enables the refrigerator to increase consumption to maintain refrigeration capacity as normal degradation of the refrigerator efficiency occurs during refrigerator operating life. The rate of helium consumption also varies with the temperature of the cryogenic refrigerator. As the cryogenic refrigerator becomes colder, a greater supply of helium is required to continue the cooling process. In a cryopumped vacuum process chamber, downtime can result in substantial economic impact, due to lost manufacturing time. Accordingly, the capability to rapidly achieve and maintain cryopumping temperatures is beneficial.
One prior art type of helium distribution is described in U.S. Pat. No. 5,775,109, entitled “Enhanced Cooldown of Multiple Cryogenic Refrigerators Supplied by a Common Compressor,” filed Jan. 2, 1997 and assigned to the assignee of the present application, incorporated herein by reference in its entirety. This patent suggests individually monitoring the temperature of each of a plurality of cryopumps to control the speed of each displacer drive motor when a cryopump attains a triggering temperature. As the refrigerators in cryopumps require varying amounts of helium depending upon the operation currently being performed, regulating the drive motor speed can reduce or increase the helium supply accordingly. In this system, each cryopump monitors temperature and controls the drive motor speed accordingly.
Frequently, however, a common helium supply manifold supplying a plurality of cryopumps is capable of supplying more helium than required by all of the cryopumps. Excess helium which is not required by the refrigerators to maintain adequate refrigeration can waste power and other resources required to maintain the helium refrigerant supply. Conversely, insufficient helium will result in inadequate refrigeration by the refrigerators and, potentially, loss of vacuum performance by cryopumps.
It should be noted that the aforementioned problems apply to cryogenic refrigerators as well as cryopumps. These refrigerators may be used in a wide variety of cooling applications including but not limited to high temperature superconductors (HTS), semiconductor manufacturing, processing and storage of biological samples, MRI imaging, and instrumentation cooling.